Hi Sven, ok thanks I think I understand better now.

Surfaces that barely graze each other and are also tangent can tend to be more difficult in surface/surface intersection calculations.

It will be easier for Trim to work on your cyan plane if you select the edges as the cutting objects rather than the full object, like this:



Then it should be cut up ok:



Also another way you can do it is to draw in 2 connecting lines like this:


Then select the lines and edges like this:



And then Construct > Planar can make a trimmed plane there:



- Michael

Thanks for explanation Michael. I realize that there are other solutions, just figured I had to be doing something wrong given the symmetry of model and it working one side, but not the other.

Thanks for the models vic.

Sven